In  megger earth tester current and potential coils of the ohmmeter are supplied with _______ and the testing circuit is supplied with _______

In  megger earth tester current and potential coils of the ohmmeter are supplied with _______ and the testing circuit is supplied with _______

Right Answer is:

DC, AC

SOLUTION

The earth tester is a special type of ohmmeter which sends ac through earth and de through the measuring instruments as shown in Fig. The direction of flow of current in the ground keeps on alternating due to current reverser whereas current direction in the two crossed coils of ohmmeter remains the same due to potential reverser. The current reversal and potential reversal are mounted on the main shaft of the hand-driven dc generator.

The ohmmeter consists of two coils mounted at a fixed angle to one another on a common axle and moving in the field of a Permanent Magnet (PM). A current proportional to the total current flowing in the testing circuit passes through the Current Coil (CC), while the Potential Coil (PC) carries a current proportional to the potential drop across the resistance under test. The cross-coil system makes the instrument a true ohmmeter giving readings in ohms that are independent of the applied voltage, speed of rotation, and contact resistance of the ground rods.

Direct current from the generator passes through the current coil of the ohmmeter to a rotating current reverser driven from the generator handle. The alternating current is thus delivered to the current terminals (the common terminals C1 and C2) of the instrument which are connected to the contact under test and to the “current” temporary earth connection. The potential coil of the ohmmeter obtains its supply from the potential terminals (common terminal P1 and the terminal P2), the latter being connected to the ‘potential’ temporary earth connection.

Since this supply is taken from the “soil” section of the current circuit and is, therefore, alternating, it must be made unidirectional before passing through the potential coil. A commutator mounted on the same shaft as the main current reverser and synchronized with it, is, therefore, interposed as a rectifier between the potential terminals and potential coil.

In this manner, both the current and potential coils of the ohmmeter are supplied with direct current. But the ‘soil’ section of the testing circuit is supplied with alternating current.

In this manner, both the current and potential coils of the ohmmeter are supplied with direct current. But the ‘soil’ section of the testing circuit is supplied with alternating current.

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